1. Field of the Invention
The present invention relates to a machine tool including a double column frame extending horizontally from side to side as viewed in front elevation and a movable unit having a tool rest and movable horizontally from side to side along the double column frame.
2. Description of the Related Art
Japanese laid-open patent publication No. 2006-263861 discloses a machine tool including a double column frame extending from side to side as viewed in front elevation and a movable unit having a tool rest and movable from side to side along the double column frame. The double column frame comprises a pair of horizontally spaced columns, an upper beam interconnecting the upper ends of the columns, and a lower beam interconnecting the lower ends of the columns. The double column frame can be manufactured as an integral casting as it has a small horizontal length.
The upper and lower beams have respective upper and lower guide rails. The movable unit is supported and guided by the upper and lower guide rails for horizontal movement from side to side along the double column frame.
A machine tool disclosed in Japanese laid-open patent publication No. 2003-311565 includes a movable unit having a tool rest and movable from side to side in the X-axis directions shown in Japanese laid-open patent publication No. 2003-311565. The movable unit is supported and guided by a pair of guide rails mounted on a bed and is movable from side to side by a ball screw mounted on the bed.
The double column frame of the machine tool disclosed in Japanese laid-open patent publication No. 2006-263861 can be manufactured as an integral casting as it has a small horizontal length. Since an upper beam of the double column frame is part of the integral casting, the upper beam is almost free of the tendency to flex downwardly by gravity. As a result, the upper guide rail is essentially free from deformations, allowing the tool rest of the movable unit to machine the workpiece highly accurately.
If, however, the double column frame is of a horizontally elongate shape and is made of steel, then the horizontally extending upper beam tends to flex downwardly by gravity. The upper guide rail is also liable to flex downwardly, and the movable unit as it moves horizontally is adversely affected by the deformation of the upper guide rail. As a result, it is difficult to move the movable unit stably and smoothly at a high speed from side to side, and the accuracy with which the tool rest machines the workpiece is lowered.
The movable unit of the machine tool disclosed in Japanese laid-open patent publication No. 2003-311565 is moved from side to side on the bed. The machine tool has no double column frame. As the ball screw is mounted on the bed, the ball screw applies horizontal forces to only a lower portion of the movable unit. No ball screw is mounted on an upper portion of the movable unit.
Consequently, since forces applied from the ball screw to the movable unit to move from side to side are localized on the movable unit, it is difficult to move the movable unit stably and smoothly at a high speed from side to side.